A RKE system is an electronic lock that controls access to a building or vehicle without using a traditional mechanical key. When the driver approaches the building or vehicle, a secure wireless communication between a key fob and a control unit authenticates the key fob. Bi-directional wireless communication authenticates the key fob and the vehicle in both one-way and two-way radio frequency (RF) systems. In one-way RF systems a LF downlink serves to wake up the key fob and to receive commands and data for the authentication process. The key fob then sends the response to the vehicle using a RF uplink. In two-way RF systems the LF downlink only serves to wakeup the key fob and to establish the RF uplink or downlink. The bi-directional RF link handles the communication during the authentication process. Building or vehicle LF antennas (coils) detect the key fob location and determine if the key fob is inside or outside the building or vehicle cabin. The position and the number of antennas can be adapted to any type of building or vehicle.
Localization is an important feature of an RKE system. Localization detects if the key fob is near the building or vehicle and if the key fob is inside or outside the building or vehicle. A car has typically four to six LF antennas. These produce an LF magnetic field covering both the car interior and the car's vicinity. The key fob measures the LF signal level during the communication with the control unit. In some RKE systems, the key fob acquires the received signal strength indication (RSSI) and sends it back to the control unit, which analyzes the RSSI to determine the position of the key fob. As the spatial orientation of the key fob is unknown, the key fob uses three discrete antenna coils or one three-dimensional (3D)-coil to determine the x-, y- and z-axes. The RSSI measurement accuracy depends on the hardware device and on the precise calibration of key fobs during end-of-line manufacturing. In some RKE systems, all three axes are measured simultaneously which reduces the overall RSSI measurement time.
In some RKE systems, the antennas in the key fob are tuned by a drive circuit that is coupled to the antenna (coil) by a MOS switch (e.g., NMOS transistor). The gate of the MOS switch is controlled by a gate drive circuit. A high gate drive voltage is often needed to properly turn on the MOS switch. Because the key fob runs on a low voltage power supply (e.g., a battery), some conventional key fobs use a voltage multiplier (e.g., a charge pump) to generate the high gate drive voltage. Unfortunately, this approach is not feasible in an extremely low power design, because the voltage multiplier needs a significant amount of current to operate.